This invention relates generally to magnetic resonance spectroscopy, and more particularly, the invention relates to volume spectroscopy in which parasitic signal interference is reduced.
Volume localized magnetic resonance spectroscopy has become a useful and routine clinical tool especially for the detection of abnormalities which lead to diffused chemical changes in the brain. Several techniques are know for directly exciting spins of metabolites such as choline, creatine and NAA in a volume of interest and achieving three-dimensional selection including use of stimulated echoes and the use of Carr-Purcell echoes. These techniques obtain a localized spectrum in a single scan. For example, point resolved spectroscopy (PRESS, see U.S. Pat. No. 4,480,228) uses a three pulse sequence with each pulse being frequency selective. Many important clinical applications of proton magnetic resonance spectroscopic imaging, MRSI, are based on phase encoding of a restricted volume of excitation. Typically, the volume excitation is achieved using PRESS, which takes advantage of three orthogonal slices in the form of a double spin echo to select a specific region of interest.
A strong water signal is present in the metabolite signals and is used to correct for phase and frequency errors due to extraneous effects. However, coherent side bands of residual water can interfere with the measurement of metabolites in localized proton spectroscopy. Typically, low frequency distortions of the metabolic and residual water signals are corrected using an unsuppressed water reference, but harmonic distortions above 20 Hz are in the range of the metabolite frequencies and are necessarily not corrected for by this approach. There are a number of potential sources for parasitic side bands including acoustic and eddy current coupling effects. The side bands which cause problems are those which are coincident with the in vivo proton chemical shift frequency range (30 Hz to 220 Hz at 1.5 T), have time constants of greater than 10 ms and have peak to peak magnitudes of greater than 0.2%. These side bands depend on magnet and gradient system, and are dependent on pulse sequence and TE. In typical probe-p (PRESS) spectra at TE 35, these side bands are observed throughout the spectral region of interest and range in size from 0.1% to over 1.0%. Typical decay constants are such that they do not pose a serious problem in the TE 144 spectra. They also do not present a problem if water is completely suppressed. This condition is limited, however, by the constraints it puts on water suppression automation, and more importantly, by the need for a relatively strong residual water signal to correct for temporal (frame to frame) phase and frequency errors due to very long time constant eddy current effects, Bo field drift and patient motion. Thus, there is a need to eliminate these side bands from an otherwise useful water signal, without impacting SNR.
Heretofore, a number of line shape correction method which have been used to correct for frequency independent line shape errors but in these methods reference signal is extracted and divided by an ideal signal and then used as a multiplier to correct the raw spectrum. This approach is highly sensitive to the method by which the reference line shape is extracted and often introduces additional noise.